Iridium-Catalyzed Double Convergent 1,3-Rearrangement/Hydrogenation of Allylic Alcohols

Enantioconvergent catalysis has the potential to convert different isomers of a starting material to a single highly enantioenriched product. Here we report a novel enantioselective double convergent 1,3-rearrangement/hydrogenation of allylic alcohols using an Ir-N,P catalyst. A variety of allylic alcohols, each consisting of a 1:1:1:1 mixture of four isomers, were converted to the corresponding tertiary alcohols with two contiguous stereogenic centers, in up to 99% ee and 99:1 d.r. DFT calculations, and control experiments suggest that the 1,3-rearrangement is the crucial stereodetermining element of the reaction.


■ INTRODUCTION
Stereoconvergent catalysis is a green and sustainable approach that converts racemic or isomeric mixtures of starting materials to a single enantioenriched product. So far, enantioconvergent catalytic processes are involved in various dynamic kinetic resolution (DKR) reactions as well as in a few recently reported enantioconvergent hydrogenations (Scheme 1). 1 For the DKR processes, a racemic starting material can be converted to enantiomerically enriched products with up to 100% yield and has been well investigated in the past halfcentury. 2 For enantioconvergent hydrogenations, the E and Z olefins, which normally generate opposite enantiomers, 1 are hydrogenated into the same enantiomerically enriched product independent of the double-bond geometry. Till date, no method exists that can convert multiple isomers (more than two isomers) to a single enantiopure product.
Enantioenriched alcohols, especially tertiary alcohols and their derivatives, provide valuable feedstock for the synthesis of bioactive natural products and pharmaceuticals. 3 Consequently, their preparation has attracted great attention from synthetic chemists, and the development of efficient preparative methods has become a high priority goal in organic synthesis. In this regard, asymmetric hydrogenation of allylic alcohols is a direct and efficient way to prepare chiral alcohols. Over the past few years, several works have been reported on the Ir-catalyzed hydrogenation of di-/tri-substituted primary/ secondary allylic alcohols with excellent enantioselectivity. 4 However, quaternary alcohols are for obvious reasons not directly accessible via asymmetric hydrogenation. Iridiumhydride complexes are considered as potential Lewis acids and/or Brønsted acids in certain cases. 5 In 2010, Burgess reported that many N,P-based Ir-complexes have a Brønsted acidity similar to acetic acid. 6 In earlier investigations, the Irhydride species was found to be acidic, and in some cases, it has the capability to cleave the allylic alcohol C−O bond ( Figure 1a). 7 Exploiting this acidic feature, an Ir-N,P-catalyzed hydrogenative DKR process was discovered and the mecha-   nistic studies showed that the acid-assisted isomerization was the crucial element of the reaction (Figure 1a (2)) 4 This reactivity pattern was also extended later to the deoxygenation of racemic alcohols to produce chiral alkanes (Figure 1a (3)) 8 The regioisomers of allylic alcohols have different thermodynamic stabilities due to the diverse extent of conjugation, substitution, and steric hindrance (Figure 1b). It has been reported that in the presence of Brønsted acids or transition metal complexes, allylic alcohols could be transformed to more stable isomers via 1,3-rearrangement. 9 However, the reported methods are usually limited to di-or tri-substituted allylic alcohols, and to obtain geometrically pure products (E or Z), geometrically pure starting material is always required.
Considering the acidic nature of the Ir-N,P catalyst, the possible 1,3-rearrangement of allylic alcohols and the efficiency of enantioconvergent catalysis led us to investigate a possible double convergent 1,3-rearrangement/hydrogenation of allylic alcohols ( Figure 1c). As shown in Figure 1c, a racemic, 1:1 E/ Z mixture of a tetrasubstituted allylic alcohol was synthesized and subjected to hydrogenation. As the direct hydrogenation of a tetrasubstituted olefin is slow, an isomerization toward the trisubstituted allylic alcohol intermediate took place, followed by hydrogenation to an enantiopure tertiary alcohol. After optimization of the reaction, an impressive double stereo-and enantioconvergent catalytic process was observed, in which all four isomeric allylic alcohols were converted into a single enantiomer in a single step. During that study, we sometimes observed carbocation formation in the allylic position. For instance, the attempted hydrogenation of a diphenyl-substituted allylic tertiary alcohol (Figure 1a (1)) led to a Friedel−Crafts product instead of the expected hydrogenation product.
These results compelled us to prepare a new substrate 1a, which was thought to be reduced to the saturated 1H-indane via a trisubstituted 1H-indene intermediate (Figure 2, top). Interestingly, when the hydrogenation of 1a was attempted, only the tertiary alcohol 4a was obtained with 80% yield and 94% ee when Cat I was used (Figure 2, bottom). In this case, the hydroxyl group was shifted via 1,3-rearrangement to form a trisubstituted allylic alcohol, which then underwent asymmetric hydrogenation in a highly enantioselective manner.
Optimization of the Reaction Conditions. The stereoselective transformation of 1a into 4a suggested that the 1,3rearrangement/hydrogenation cascade held potential to become a useful reaction. To further evaluate the efficiency of this methodology, an isopropyl group was introduced, giving rise to the asymmetric olefin 2a. In the first attempts, a 1:1 E/Z mixture of racemic 2a was hydrogenated using Cat I in CH 2 Cl 2 under 10 bar of H 2 . Initially, only 20% of the product 5a was observed together with a large number of byproducts obtained from deoxygenation, dimerization (ether formation), and their respective hydrogenated products. Screening of different solvents (Table 1, entries 1−4) revealed that PhCl resulted in a more selective reaction, giving 5a in 80% yield (Table 1, entry 4) and excellent d.r. and ee (97/3 d.r., 98% ee). Further optimization showed that PhF was the best solvent in terms of reactivity, enantioselectivity, and diastereoselectivity (87% of product, 97/3 d.r., 98% ee, Table 1, entry 5). Subsequently, a catalyst screening established complex Cat II to be the most efficient and stereoselective catalyst for this reaction (Table 1, entry 6). To conclude, the optimal conditions for good yield, good enantioselectivity, and stereoselectivity are 1.0 mol % of Cat II, 10 bar of H 2 pressure, and PhF as the solvent (entry 6). Notably, in this case, a 1:1 E/Z mixture of the racemic alcohol, which consists of four different isomers in total, is converted to a single diastereomer, enabling an efficient double convergent 1,3-rearrangement/hydrogenation reaction.
Substrate Scope. With the optimized conditions in hand, the substrate scope of the asymmetric 1,3-rearrangement/ hydrogenation of allylic alcohols was examined. First, we evaluated the hydrogenation of various 2,3-dimethyl-1-phenylbut-2-en-1-ol derivatives (Table 2a). Substrates bearing different substituents on the phenyl ring were all successfully hydrogenated to give the desired tertiary alcohols 4a-4h. Both electron-donating and electron-withdrawing substituents gave good to excellent isolated yields (84−98%) and excellent enantioselectivity (90−99% ee). Interestingly, the orthomethyl-substituted substrate 33g gave 99% ee (ortho substitution of styrenic derivatives normally results in lower enantioselectivities in asymmetric hydrogenations). The naphthyl group (4h) was also tolerated (86% yield, 90% ee). Further investigation was focused on more challenging substrates having unsymmetrical olefins that exist in E/Z mixtures and result in two contiguous stereogenic centers.
Substrate (E/Z)-2a was synthesized by simply adding Grignard reagents to E/Z mixtures of the corresponding aldehydes (∼1:1 ratio). The resulting product mixtures are very difficult to separate by column chromatography. However, when subjecting these 1:1:1:1 mixtures of isomers directly to the 1,3-rearrangement/hydrogenation protocol, all isomers were readily converted to a single, optically pure tertiary alcohol in excellent isolated yield, enantioselectivity, and stereoselectivity. First, substrates bearing different alkyl groups on the double bond, ranging from ethyl, isopropyl, and cyclohexyl groups were evaluated. All of these were hydrogenated efficiently (5a-d) in good yield (85−91%) and enantioselectivities (90−99% ee). Substrates with a less bulky substituent (Et, 5b-c) gave lower diastereoselectivity. The i Pr group resulted in the highest selectivity and afforded 90% of isolated yield with 99% ee and 98.5:1.5 d.r. Substrates bearing different functional groups on the phenyl ring were also investigated. Electron-donating or electron-withdrawing substituents on the aromatic ring were tolerated and gave good to excellent isolated yields (80−96%), excellent enantioselectivity (90−99% ee), and outstanding stereoselectivity (96/4−99/1 d.r.) ( Table 2, 5e-l). The substrate with the naphthyl group (5m) resulted in similar stereoselectivity.
Usually, the 1,3-rearrangement of allylic alcohols requires an aryl group next to the C-O bond to give stable conjugated allylic alcohols. 10 Moreover, the hydrogenation of allylic alcohols usually requires substrates bearing at least one aryl group on the γ-carbon in order to give high enantioselectivities. Gratifyingly, this protocol was found to be independent of the Scheme 2. Hydrogenation of Trisubstituted Allylic Alcohols via 1,3-Rearrangement a a aliphatic or aromatic nature of the substituents, and various aliphatic substrates 3 were efficiently and selectively hydrogenated under mild conditions. Interestingly, the H 2 pressure could be reduced to 1 bar in these cases. The benzylsubstituted allylic alcohol 3a delivered the desired product in 85% isolated yield with 98% ee and 98:2 d.r. Next, substrates having various alkyl groups from short to long aliphatic chains gave satisfactory results (89−99% ee, 94:6−99:1 d.r., Table 2, 6b-g). Generally, substrates bearing longer chains afford higher enantioselectivities.
A variety of tetrasubstituted allylic alcohols were successfully converted to enantioenriched tertiary alcohols via 1,3rearrangement/asymmetric hydrogenation. We then proceeded to investigate the hydrogenation of some trisubstituted allylic alcohols (Scheme 2). These are usually directly hydrogenated to the product; therefore, a competition between 1,3rearrangement and direct hydrogenation will occur during the process. Interestingly, cyclohex-1-en-1-yl(phenyl)methanol 7a was converted to the desired secondary alcohol with 88% ee. The major anti-diastereomer (8a) was purified by column chromatography and was isolated in 35% yield. Another interesting result was observed when a methyl group was introduced on the cyclohexenyl ring (7b). In this case, the chirality was transferred from the remote carbon and resulted in 88% ee and 30% isolated yield of the major diastereomer. These results show that some trisubstituted olefins are also tolerated in the 1,3-rearrangement/hydrogenation although competitive direct hydrogenation byproducts are also generated.
Finally, we investigated if the presence of an internal nucleophile could intercept the allylic carbocation formed in the reaction. The transformation was performed on substrate 9, having an additional hydroxyl group on the ortho position of the ring, which afforded 2H-chromene 10 as the product. Under optimized conditions, the intramolecular cyclization followed by asymmetric hydrogenation resulted in a very efficient kinetic resolution, and the remaining 2-chromene could be isolated in 40% yield with 99% ee (Scheme 3). the active catalyst under hydrogenation conditions and generate iridium hydrides, [(N,P)*Ir(H) 2 (solv) 2 ]BAr F , resulting in the liberation of HBAr F . The Ir-hydrides and formed Brønsted acid could be responsible for the 1,3-rearrangement besause Brønsted acids, transition metal complexes, and even neutral H 2 O 11 have been reported to catalyze the 1,3rearrangement. When 1a was subjected to the pre-activated catalyst, a fast rearrangement took place and the racemic (E)-2,3-dimethyl-4-phenylbut-3-en-2-ol (E-11a) was formed in a 99:1 E/Z ratio (Scheme 4).
To further understand this transformation, DFT calculations for an aryl substituted allylic alcohol and an alkyl substituted allylic alcohol were conducted (Scheme 5). The minimized conformations of the starting materials and possible rearrangement products are shown in Scheme 5. When the smallest substituent in the allylic position is nearly eclipsed with the double bond and the vinylic hydrogen, the 1,3-strain (A (1,3)) was minimized (θ = 9°). The E-isomer for the 1,3rearrangement product was the most stable configuration (E-11a, E-11i). The other possible aromatic allylic alcohol conformations had energies of >4.58 kcal·mol −1 , and the energies of the aliphatic example were >2.30 kcal·mol −1 .

Control Experiments.
To better understand the features of the 1,3-rearrangement of the allylic alcohol, several control experiments were conducted. First, substrate 2a was added to complex Cat II in the absence of hydrogen. Under these conditions, the Ir-N,P complex can be regarded as a Lewis ; however, no product was observed in this case (Table 3, entry 1). Next, the experiment was carried out using preactivated Cat II together with 10 mol % K 2 CO 3 to remove HBAr F (Table 3, entry 2). No conversion was observed, which suggests that the neutral iridium trihydride species alone does not catalyze the 1,3-rearrangement. Finally, when substrate 2a was treated with 10 mol % Brønsted acid (HBAr F. 2Et 2 O), a complex mixture of products was obtained (Table 3, entry 3), which excluded the possibility of the acid alone catalyzing the 1,3-rearrangement. Proposed Pathway I n 2021, we reported on an efficient kinetic resolution protocol for a variety of trisubstituted allylic alcohols via Ir-N,P-catalyzed asymmetric hydrogenation. In that case, an interaction between the hydroxyl group and iridium hydride was proposed to control the stereochemical outcome of the reaction. DFT calculation showed that a hydrogen bond between the alcohol and the iridium center is responsible for the enantioselectivity.
When the larger group on the carbinol is pointing toward the ligand backbone, the steric interactions disfavor the hydrogenation (Figure 3, mismatched enantiomer). Contrarily, there is a match for the other enantiomer, where the hydrogen bonding results in the smaller substituent pointing toward the catalyst (Figure 3, matched). This difference in steric clash resulted in an energy difference of 3.0 kcal·mol −1 for the two transition states. 4d Based on these control experiments, we propose that the OH group is cleaved with the assistance of the acidic Irhydride species. This results in 1,3-rearrangement taking place and forming the thermodynamically more stable isomer (Scheme 6, step 1). As mentioned above, the bulkier group

■ CONCLUSIONS
To conclude, we have developed an efficient catalytic system for a doubly convergent 1,3-rearrangement/hydrogenation of allylic alcohols. A variety of allylic alcohols consisting of a 1:1:1:1 mixture of four isomers were converted to the corresponding tertiary alcohols with up to 99% ee and 99:1 d.r. In addition, DFT calculations and control experiments agreed with the outcome of the 1,3-rearrangement. Finally, a rationale explaining the origin of selectivity in this enantioconvergent hydrogenation was also proposed.
Experimental procedures, characterization data of new compounds, separation of chiral products, NMR spectra of new compounds and computational details (PDF)